This presentation will discuss efficient PCB material selection. Before selecting begins, there are many factors to consider. Make sure material characteristics fit your specific board requirements and end application. Today, we will focus on the dielectric properties, cost, and manufacturability of materials suitable for high-speed PCB designs.
One of the main problems we face when manufacturing PCBs is the PCB designers’ frequent over-reliance on the material datasheets. Don’t get me wrong, data sheets provide designers with a thorough description of a material’s electrical properties, and electrical properties are the primary consideration for high-speed applications. However, the data sheets fall short when taking into account various real-world manufacturing concerns, and real-world manufacturing concerns matter because they impact yield and cost.
How do I choose a PCB material?
5 Basic PCB Material Categories
Regarding PCB material selection for high-speed PCBs, what are the primary concerns that must be concerned in regards to one, manufacturability, and two, cost? Let’s take a look at this chart. For convenience, we’ve classified important materials into buckets based on the material’s signal loss properties.
On the left, we have materials like FR-4. These are your standard, everyday materials, such as materials from China like Nan Ya, and materials from the US, like Isola. FR-4 are standard materials that can be used in any application, but they are also the lossiest laminates. It can also have a plethora of other electrical and mechanical issues, and if you’re having issues with FR-4 material, shoot me an email. I would love to help.
As we move across the chart, you can see the less lossy, higher speed application PCB materials. Rogers 4350 performs similarly to Megtron 6 and Itera, so these are the materials you should consider when you need that level of performance.
How to choose PCB laminates?
PCB material selection depends upon various characteristics of the dielectric material used. These properties include :
- Glass transition temperature (Tg) – the temperature at which the material transforms from a solid state to a viscous state is called glass transition temperature. This is a critical parameter for PCB material selection.
- Thermal decomposition temperature (Td) – the temperature at which the material decomposes chemically is called thermal decomposition temperature.
- Dielectric constant (Dk) – the speed at which an electrical signal travels in the dielectric medium is called the dielectric constant. This is a critical parameter for PCB material selection.
- Loss tangent (tanδ) – the power loss of a signal as it passes through a transmission line on a dielectric material is called the loss tangent.
- Coefficient of Thermal Expansion (CTE) – the change in dimensions of a material as a response to a change in temperature is called the coefficient of thermal expansion
- Thermal conductivity (Tc) – the property of a material to conduct heat is called thermal conductivity.
- RoHS compliance – the minimization or elimination of hazardous substances such as lead is measured as compliance to RoHS standards.
Signal Loss and Operating Frequency
So, the questions arise. What PCB material properties account for the difference in the PCB electrical performance, and how do these differences affect the PCB material cost? As it turns out, there are three main factors to evaluate when it comes to material performance for high-speed PCB designs. What is the signal loss at the operating frequency? Should you be concerned about the weave effect, and how easy is the material to manufacture your stack-up in construction?
First, let’s take a look at the relationship between signal loss and operating frequency. As you can see from the graph, there’s a direct correlation between signal loss and frequency. At the same time, we can also see that certain materials are less lossy than others. This was the basis we used to create our material classification bucket on the previous chart. This graph shows which materials could possibly perform better electrically at higher speeds.
Next, let’s compare the direct cost based on our material classifications. As you can see from the chart, less lossy materials cost more. You’ll have to decide what materials work best for your specific project. As you can see, the Rogers 4350B material is higher than that of Megtron 6 or Itera, even though electrical performance is similar. In the microwave category, the Taconic RF-35 is about 30% less expensive for the same performance as other materials in this category.
Non-PTFE (Polytetrafluoroethylene/Teflon) Materials
Let’s do a deeper dive into the non-PTFE materials. We’ll come back to the PTFE materials in a bit. Now, all of these materials perform somewhat similarly and at somewhat similar costs, but what justifies the cost differences, and what is the advantage of working with higher cost materials?
First, we must understand material construction, and the effects of glass on characteristic impedance must also be understood. One way to achieve this is by understanding the weave effect and the different types of glass cloth. As you can see, different glass construction will effect DK distribution. A board with a loose weave will have greater variation in board thickness and greater variation in DK distribution. However, a tight weave will have a more consistent board thickness and more even DK distribution. The effective DK of the material remains the same as the signal traverses the dielectric.
What’s really important to note from a manufacturing perspective is that a board with a tighter weave is easier to manufacture. When the glass weave is more consistent, mechanical laser drilling also becomes more consistent.
Aside from the glass weave, there are two other types of glass to choose from, Si glass or E-glass. E-glass is the predominant glass type. It varies thickness between 1.3 mils to 6.8 mils. Looking at the chart, you can see the DK of the E-glass at 5 gigahertz is 6.5, while the DF is .006. Now, Si glass is much purer than E-glass, and as a result, the DK of 5 gigahertz for the Si glass is 4.5 and the DF is .004. The cost of the laminate compared to E-glass is about 15% higher, well worth it, if you ask me.
How much does a sheet of laminate cost? – Indirect costs
Now, let’s look at some indirect costs that have to be considered during PCB material selection. It’s very important to remember that we can’t just look at the cost of the material by itself. In fact, most of the overall cost doesn’t come from direct material costs at all, but from the PCB process cost associated with that PCB material.
One of the key factors that impact cost in manufacturing is lamination. Materials in the medium, low, and extremely low loss categories generally manufacture the same way as FR-4, although some materials are more dimensionally stable than others, and some materials are easier to laser.
On the other hand, materials in the RF microwave category do not register like non-PTFE category materials, and thus become very difficult to manufacture, especially in multi-layer stack-ups. This difficulty is primarily because PTFE materials have a problem of stretching. Usually, we use scrubbing to prep materials before lamination, but for this category of materials, scrubbing is a problem. We have figured out for sure how to achieve reliable adhesion after the lamination, but it’s still difficult.
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Key manufacturing considerations
So, next, let’s discuss key manufacturing considerations when dealing with hybrid PCB stack-ups. First, make sure all the materials in your hybrid stack-up are compatible with your lamination cycle. Some materials need higher temperatures and pressures than others in the lamination process. Before you submit your design, check your material data sheets to confirm compatible materials are being used.
The second consideration in hybrid stack-ups is drill parameters for proper hole formation. Feeds and speeds of the drill bits vary based on materials in the stack-up. If you have a stack-up which is a pure construction, meaning it’s all the same material, versus a hybrid construction, the feeds and speeds have to be adjusted. For example, certain settings generate a lot of heat and if the material cannot withstand the heat, there can be some deformation. You should also take into account that different materials drill differently. Rogers, for example, wears the drill bits down faster and thus impacts the cost.
After drilling and before cuposit, there is hole wall preparation. Different materials require different plasma. A dirty little secret among manufacturers is that not all of us refine our process per the material. There can be process guidelines per general category, but for absolute reliability and on-time delivery, the manufacturers should be refining their process per material.
PCB Stack-Up Guidelines for Mixed Materials
Next, we’ll review three stack-ups and go over some basic stack-up guidelines for mixed materials. Stack-up number one is a pure Rogers stack-up using Rogers 3000 materials. It is a multilayer construction that requires longer dwell times at higher temperatures. This lamination process is known as fusion bonding. Only a select few manufacturers, like Sierra Circuits, have the equipment and the expertise to perform this operation.
Stack-up number two is a hybrid stack-up using Rogers and Isola materials. Designers use this method to save on material cost and to aid in the manufacturability of multilayer stack-ups. Rogers is not suitable for sequential lamination process, and there are other material vendors like Taconic and Isola who make materials that perform similar to Rogers and do not have these limitations. In the past, it’s been difficult to control the press-out thickness of these B-stage materials. Now, with better equipment, better process controls, customers can expect consistency and reap the benefits.
Third and last is a stack-up consisting solely of Taconic materials. These materials, although based on glass cloth, have similar performance to Rogers materials and are much easier to manufacture. With glass cloth, materials also become dimensionally stable.
Hybrid Stack-Up Guidelines
Now, let’s discuss some hybrid stack-up guidelines. We recommend the following when dealing with a hybrid construction. Use the high-performance material as the core. Laminate with FR-4 prepreg. Balance the FR-4 portion, and don’t use a high Tg dielectric or bonding film with a lesser Tg material.
So, there you have it. We reviewed how to select high-speed materials based on performance and cost, including manufacturability, and I’ve reviewed three relatively complex stack-ups.
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